Mitochondrial malate dehydrogenase, decarboxylating ("malic" enzyme) and transhydrogenase activities of adult Hymenolepis microstoma (Cestoda).

Adult H. microstoma mitochondria catalyzed a malate dehydrogenase, decarboxylating ("malic" enzyme) activity. This "malic" enzyme was found as a soluble component of the mitochondrion, was specific for NADP, and required a divalent cation with Mn++ ion yielding the greatest activity. The H. microstoma "malic" enzyme could fulfill the need for generating intramitochondrial reducing equivalents required for electron transport. The H. microstoma mitochondria also exhibited an NADPH:NAD transhydrogenation reaction. The electron transport system of this cestode was apparently specific for NADH both in terms of the rotenone-sensitive oxidase and fumarate reductase systems. Electron transport-associated NADPH oxidation was increased markedly with the addition of NAD to the system. Coupling of NADPH utilization to fumarate reduction, in the presence of NAD, was apparent under conditions of reduced oxygen tension. This was consistent with the presence of the NADPH:NAD transhydrogenase which catalyzed a transfer of reducing equivalents from NADPH to NAD, producing NADH for electron transport function. The data presented suggest that H. microstoma mitochondria can engage in an anaerobic, electron transport-associated production of succinate, and presumably concomitant phosphorylation. Malate may serve as the mitochondrial substrate supplying reducing equivalents for electron transport via the activity of the "malic" enzyme coupled to the NADPH:NAD transhydrogenase. In addition to the NADPH:NAD transhydrogenase activity, H. microstoma mitochondria catalyzed an NADH:NAD transhydrogenation.